For high speed image sensors, a global shutter can be used to capture fast-moving objects. A global shutter typically enables all pixels in the image sensor to simultaneously capture the image. For slower moving objects, the more common rolling shutter is used. A rolling shutter normally captures the image in a sequence. For example, each row within a two-dimensional (“2D”) pixel array may be enabled sequentially, such that each pixel within a single row captures the image at the same time, but each row is enabled in a rolling sequence. As such, each row of pixels captures the image during a different image acquisition window. For slow-moving objects the time differential between each row generates image distortion. For fast-moving objects, a rolling shutter causes a perceptible elongation distortion along the object's axis of movement.
To implement a global shutter, storage capacitors or storage transistors can be used to temporarily store the image charge acquired by each pixel in the array while it awaits readout from the pixel array. When a global shutter is used, leakage current of the stored charge from a captured image increases because of the longer time periods in which the charge is held. Because the rows are read out sequentially, leakage for some rows can be greater than leakage for other rows. Accordingly, the leakage can cause the image to vary by rows with respect to displaying uniform colors. When using a storage transistor, the image charges are further corrupted by large leakage current from surface states beneath the gate of the storage transistor.
FIG. 1A illustrates a conventional complementary metal-oxide-semiconductor (“CMOS”) imaging pixel 100 with a global shutter. Imaging pixel 100 comprises a shutter transistor 110, photodiode 120, transfer transistor 130, storage transistor 140, output transistor 150, reset transistor 160, amplifier transistor 180, and row-select (RS) transistor 190.
FIG. 1B illustrates operation of imaging pixel 100. Shutter transistor 110 is deactivated with a global shutter signal to acquire an image signal or charge within photodiode 120. While the image charge is being acquired, imaging pixel 100 resets floating diffusion (FD) node by activating reset transistor 160 while shutter transistor 110 is disabled. Then, output transistor 150 is toggled on and off while storage transistor 140 remains deactivated. Reset transistor 160 is then disabled and storage transistor 140 enabled to prepare for image transfer from photodiode 120. Image transfer from photodiode 120 to storage transistor 140 is accomplished by activating transfer transistor 130 long enough to transfer all charge from photodiode 120. Transfer transistor 130 is once again deactivated in preparation for the next image acquisition window, while the current image charge is stored in storage transistor 140. The current image is then readout of each imaging pixel 100 in an array of imaging pixels line-by-line by appropriate assertion of row select transistor 190 and output transistor 150.
In imaging pixel 100, both transfer gate transistor 130 and shutter gate transistor 110 are connected to photodiode 120. Shutter gate transistor 110 typically has a device size that is similar to transfer gate transistor 130 in order to fully deplete (photo-voltaically generated) charges in photodiode 120. Additionally, transfer gate transistor 130 is normally wider than storage gate transistor 140 and larger than control gate transistor 150, reset transistor 160, amplifier 180, and row-select transistor 190 in order to fully deplete the photodiode 120. Thus, two large-size devices are normally provided for each conventional imaging pixel. Because two relatively large size devices are used for each imaging pixel, the photodiode area of each pixel is correspondingly smaller. The relatively smaller photodiode area reduces the fill factor of imaging pixel 100, which reduces the amount of pixel area that is sensitive to light and reduces low light performance.